1991 blazer evap question.

[jesse james 1]

Swapping caps would never have solved this as the caps are designed to vent to atmosphere, not prevent vacuum. Pre-EVAP they were, but you can't get those caps anywhere I've ever seen. I made my non-vented cap vented, and promptly realized filling the truck to full and getting it nose up meant I was dumping fuel, which amazingly is worse for gas mileage than burning it.

The only way the tank is collapsing is that it's not able to draw air as it should, which would be through the EVAP system.

Now that your cap is no longer functional, doing something simple like unplugging the EVAP canister after driving wont tell you anything. If unplugging the tank to EVAP line vented the tank, you'd know the canister isn't "breathing" properly.

I'll do some more experimenting on the evap side soon then, everything in new, not saying new parts can't be junk either haha. Here's the list of replaced parts associated with fuel/evap seystem so far, fuel cap, filler neck and hoses, tank, pump /sending unit assembly, PURGE valve and hoses, supply and return fuel lines from pump to Throttlebody, evap lines from purge valve to charcoal canister, charcoal canister, vacuum line from charcoal canister to throttlebody, throttlebody and air cleaner gaskets, miscellaneous vacuum lines in engine bay.

 

[dyeager535]

The EVAP system isn't complex, at all. It's literally a line from the tank to the EVAP canister, and the line to the throttle body. The canister is supposed to breathe via a charcoal filter at the bottom, but I don't presently have EVAP so I've never dug into it beyond this. Was forever before I discovered EVAP had a filter. Prior to that I was dumb and thought EVAP was also dumb, so didn't run it. Better to burn fumes than smell them I learned. Plus gasoline fumes are kinda dangerous. Who knew?

Very easy to narrow this down to tank-side of canister or engine side of canister (or canister itself). It doesnt seem like the line to canister can be bad, as I recall you cleaned it and tested that it passed air. I'd still verify that air moves freely from the canister end of that line, should be able to blow through it with the cap vented. Unlikely, but crud could theoretically be acting as a one way valve I suppose.